Introduction: The development of Open MR Scanners, such as the Signa-SP facility at Stanford has opened the door for MR-imaging during minimally invasive procedures. The goals of this project are to develop techniques for MR-guidance and monitoring of therapeutic thermal and cryogenic lesions. MR is ideally suited to image guidance during minimally invasive procedures because the inherent multi-planar capabilities allow precise positioning of ablative devices at desired targets in the body. However the real potential of MR is its ability to monitor the effects of ablative processes while they are being performed. Specifically, we are developing methods to image the heat, and ice formed during thermal, and cryogenic lesions, respectively. Methods: MR-compatible RF ablation devices are being developed through a collaboration with Radiotherapeutics corporation. An MR-compatible RF ablation system which can ablate lesions up to 3.5 cm in diameter via a percutaneous approach is under development. Phantom and animal testing has been performed to evaluate device performance. Thermocouple validated trials are underway to test the accuracy of phase-difference MR thermal maps during heating with the device in phantoms and animals. MR-compatible cryoablation systems are being developed through a collaboration with Cordis corporation. With prostate carcinoma as the initial target, device testing in tissue phantoms and a dog prostate model has been implemented to test system performance, and develop MR-guided methods for percutaneous prostate ablation. Results: Successful RF ablation has been demonstrated during MR imaging in electrolyte-doped agar phantom and animal models. Phase difference images clearly show the pattern of head deposited by the probe, and correlate well with gross lesion size and shape. Phase difference imaging has proven accurate within 3 degrees C, within 4 mm of the probe, an a agar phantom. Conclusions: Magnetic resonance imaging with the Signa-SP systems holds great promise for image guidance and monitoring during minimally invasive thermal and cryo therapies. Future research will focus on refining the accuracy of MR-based temperature measurements, and ice ball imaging.